Liquefied petroleum gas (LPG) is a liquefied petroleum-based or natural-gas-based hydrocarbon which is gaseous at an ambient temperature under an atmospheric pressure by compression while optionally cooling, and the main component of it is propane or butane. LPG is advantageously transportable because it can be stored or transported in a liquid form. Thus, in contrast with a natural gas that requires a pipeline for supply, it has a characteristic that it can be filled in a container to be supplied to any place. For that reason, LPG comprising propane as a main component, i.e. propane gas, has been widely used as a fuel for household and business use. At present, propane gas is supplied to about 25 million households (more than 50% of the total households) in Japan. In addition to household and business use, LPG is used as a fuel for a portable product such as a portable gas burner and a disposable lighter (mainly, butane gas), an industrial fuel and an automobile fuel.
Conventionally, LPG has been produced by 1) collection from a wet natural gas, 2) collection from a stabilization (vapor-pressure regulating) process of crude petroleum, 3) separation and extraction of a product in, for example, a petroleum refining process, or the like.
LPG, in particular propane gas used as a household/business fuel, can be expected to be in great demand in the future. Thus, it may be very useful to establish an industrially practicable and new process for producing LPG.
As a process for producing LPG, Japanese Patent Laid-open Publication No. 61-23688 discloses that a synthesis gas consisting of hydrogen and carbon monoxide is reacted in the presence of a mixed catalyst obtained by physically mixing a methanol synthesis catalyst, specifically a CuO—ZnO—Al2O3 catalyst or a Pd/SiO2 catalyst, with a methanol conversion catalyst composed of a zeolite having an average pore size of about 10 Å (1 nm) or more, specifically a Y-type zeolite, to give a liquefied petroleum gas or a mixture of hydrocarbons similar in composition to LPG.
However, the above-mentioned process does not always give a sufficiently high activity (a conversion of carbon monoxide), a sufficiently high yield of a hydrocarbon, and a sufficiently high yield of propane and butane. A yield of hydrocarbon is at most 36.0%, while a yield of propane and butane is about 26%. In another case, a yield of hydrocarbon is 35.7%, while a yield of propane and butane is about 27%.
Furthermore, a product obtained by the above-mentioned process may not have a sufficiently low carbon dioxide content. When a yield of hydrocarbon is at its highest, that is 36.0%, a yield of carbon dioxide is 33.9%. When a yield of hydrocarbon is 35.7%, a yield of carbon dioxide is 30.7%. Carbon dioxide is less useful and is hard to be reused, and therefore, it is economically undesirable to yield a large amount of carbon dioxide as a by-product.
As a process for producing LPG, “Selective Synthesis of LPG from Synthesis Gas”, Kaoru Fujimoto et al., Bull. Chem. Soc. Jpn., 58, p. 3059-3060 (1985) discloses that, using a hybrid catalyst consisting of a methanol synthesis catalyst such as a 4 wt % Pd/SiO2, a Cu—Zn—Al mixed oxide {Cu:Zn:Al=40:23:37 (atomic ratio)} or a Cu-based low-pressure methanol synthesis catalyst (Trade name: BASF S3-85) and a high-silica Y-type zeolite with SiO2/Al2O3=7.6 treated with steam at 450° C. for 1 hour, C2 to C4 paraffins can be produced in a selectivity of 69 to 85% via methanol and dimethyl ether from a synthesis gas. However, as is in the process described in the above-mentioned Japanese Patent Laid-open Publication No. 61-23688, this process does not always give a sufficiently high activity (a conversion of carbon monoxide) and a sufficiently high yield of a hydrocarbon, and the obtained product may not have a sufficiently low carbon dioxide content.
On the other hand, “Methanol/Dimethyl Ether Conversion on Zeolite Catalysts for Indirect Synthesis of LPG from Natural Gas”, Yingjie Jin et al., Dai 92 Kai Shokubai Touronkai TouronkaiA Yokousyuu, (the summaries of the 92th Catalysis Society of Japan (CatSJ) Meeting, Meeting-A), p. 322, Sep. 18, 2003 discloses a process for producing LPG, using at least one selected from the group consisting of methanol and dimethyl ether as a starting material. Specifically, a starting gas, whose composition is methanol:H2:N2=1:1:1, was passed through the two-layered catalyst layer consisting of ZSM-5 as the former layer and Pt—C as the latter layer (ZSM-5/Pt—C Series) or a mixed catalyst layer consisting of ZSM-5 and Pt—C (ZSM-5/Pt—C Pellet-mixture), under a slightly increased pressure, at a reaction temperature of 603 K (330° C.) and at a methanol-based LHSV of 20 h−1, whereby carrying out an LPG production reaction.
However, the above-mentioned process may not give a sufficiently high conversion of methanol to propane and butane. When using a ZSM-5/Pt—C Series as a catalyst layer, a conversion of methanol to a hydrocarbon is 64.0% on the basis of carbon, while a conversion of methanol to propane and butane is about 38.7% on the basis of carbon. When using a ZSM-5/Pt—C Pellet-mixture as a catalyst layer, the result is even worse; specifically, a conversion of methanol to a hydrocarbon is 20.6% on the basis of carbon, while a conversion of methanol to propane and butane is about 10.8% on the basis of carbon.
Furthermore, when using a ZSM-5/Pt—C Series as a catalyst layer, the deterioration with time of the catalyst may be generally significant and the catalyst life may not be sufficiently long. Generally, when an olefin is produced from methanol and/or dimethyl ether using a zeolite as a catalyst, the zeolite catalyst is apt to be deteriorated due to coking.